Since the beginnings of continuous casting the persons skilled in the art have been occupied with the problem of the formation of air gaps below the bath level between the strand crust and the mould wall. These gaps quite substantially reduce the heat transfer between the mould and the strand crust and cause uneven cooling of the strand crust, which leads to faults in the strand, such as rhomboid shaping, cracks microstructural defects etc. In order to create optimum contact of the strand crust with the mould wall on all sides over the whole length of the mould and thus to obtain the best possible conditions for heat dissipation, many proposals have been made, such as walking beams, the squeezing of coolant into the air gaps, mould cavities with varying conicities etc.
U.S. Pat. No. 4,207,941 discloses moulds for the continuous casting of steel strands with square cross-sections. The cross-section of a mould cavity which is open at both sides is a square with corner chamfers on the inlet side and an irregular dodecagon on the strand outlet side. The taper steadily increases towards the corner chamfers of and near the chamfers is approximately twice as large as in the central region of the mould wall. In casting with such moulds, the strand can become wedged inside the mould, causing breaking off and splitting of the strand. Also, instead of a square, a dodecagon is cast. It is difficult to dimension such moulds for different casting speeds, such as are inevitable in long sequence casting operations with many changes of ladle.
U.S. Pat. No. 4,774,995 discloses a continuous casting mould whose cross-section is larger on the inlet side, in order to receive an immersed pipe, than on the strand outlet side. As the strand passes through the mould, the thickness of the strand decreases, together with the cross-sectional area of the strand, due to deformation upon contact with the wide sides of the mould. The narrow sides of the mould diverge in the direction of travel of the strand in a manner corresponding to the reduction in thickness of the strand, so that the circumference of the strand remains substantially constant. The application of a conventional pouring spout in this casting method causes severe deformation of the strand crust on two sides of the strand, without yielding more homogeneous cooling over the whole circumference of the strand.